Where dunes migrate during deposition, they move upward (climb) with respect to the generalized depositional surface. Sediment deposited on each lee slope and not eroded during passage of a following trough is left behind as a cross-stratified

End_Page 982------------------------------

bed. Because sediment is thus transferred from dunes to underlying strata, bed forms must decrease in cross-sectional area or in number, or both, unless sediment lost from dunes during deposition is replaced with sediment transported from outside the depositional area.

Using equations that relate the amount of sediment lost by dunes to the amount gained by sets of cross-strata, we calculate that the dunes which deposited the De Chelly (Permian), Navajo (Triassic? and Jurassic), and Entrada (Jurassic) Sandstones had mean heights with lower and upper limits of 16 and 450 m, respectively. Although these calculated dune heights are surprisingly large, two kinds of field observations support the hypothesis that the dunes that deposited 10-m thick sets of cross-strata common in eolian sandstones may have been 100 m high or higher--comparable in size to large modern dunes or draas. First, many sets of eolian cross-strata are primarily bottomset beds; they rarely contain dune-crest deposits or convex-upward cross-strata deposited on lupper lee slopes, and s me sets contain sand-flow toes that pinch out near the top of the set. Second, the sand-flow layers in these eolian sandstones are thicker and laterally more extensive than in modern 10-m high dunes. The hypothesis that large dunes deposit relatively thin sets of cross-strata explains the absence in the geologic record of sets of eolian cross-strata comparable in thickness to the height of large modern dunes.

End_of_Article - Last_Page 983------------